I can see you are making a UDF for the "E/CRC erosion model" which I have also played with before.

I believe you have one major flaw in your UDF and that is how you are defining the "Brinnel Hardness" of your material (you have it as the variable Z =3.434e+8). From the paper I am assuming you got this equation from, it should be a value of 334.4 which represents Inconel 625. This was verified when I tested out the UDF model (one I created) with the paper's values experimental values. I would ALWAYS verify your erosion models with whatever lab/experimental data you can get your hands on. In my case, I was usually forced to mimic the models described in various papers and try to get as close as possible to the numbers.

If that doesn't help, there are a few things you should do in Fluent to ensure you are getting a solution that is independent of the amount of particles injected into the model or how you are tracking them.

Dear Liam,
Thanks for your reply. I think you are right.I want to try how to write the UDF about erosion rate. So I found an equation from a paper(Comparison of computed and measured particle velocities and erosion in water and air flows), random define the value of Brinnel Hardness.I have felt I can get a result similar to the Fluent. I did't realize the value of Brinnel Hardness will bring great influence.
Kind regards,
LiBo

Hey Taz,
This a 3D case.I want to kown how to write the UDF to change ersion rate when I am read an article( Comparison of computed and measured particle velocities and erosion in water and air flows).Above UDF is all my needed to change the erion rate.
I hope it can give you some help.
Cheers,
LiBo